Hurricane Otis made landfall near Acapulco, Mexico as a Category 5 hurricane early on 25 October. But around a day earlier, weather models had been forecasting that Otis would remain a tropical storm with far less destructive power.
“A nightmare situation is unfolding for southern Mexico this evening with rapidly intensifying Otis approaching the coastline,” warned the US National Hurricane Center on 24 October.
The failure to forecast Otis’s explosive intensification means that many people may have had little if any warning of what was heading their way. So why did the models get it so wrong, and can forecasters do better in the future?
The short answer is that no one knows, says Brian McNoldy at the University of Miami in Florida. “There’s still processes that can happen in a very short amount of time in a hurricane that we just don’t understand,” he says.
To be clear, for all the different models to be so far off the mark is now unusual.
“Models have improved vastly over the years in the prediction of tropical cyclone intensity when the intensification happens at an average or slow rate,” says Julian Heming at the UK’s Met Office. “However, the rapid intensification of tropical cyclones is often something models still struggle to predict with a high degree of accuracy.”
And Hurricane Otis was extremely unusual. Only one other hurricane is known to have intensified more rapidly over a 24-hour period: Hurricane Patricia in 2015. Though, Otis now holds the record for intensification over a 12-hour period.
“It is not unprecedented, but it’s completely on the tail end of what can happen,” McNoldy says.
The main factors involved in the intensification of tropical cyclones are well understood. Firstly, the ocean surface has to be very warm. This creates warm, moist air that rises. As it does, the water vapour condenses and releases latent heat, warming the air and making it rise even higher. This is what powers hurricanes.
If there is a lot of dry air near a storm, it can get pulled in and reduce the amount of condensation, depriving a storm of its power source. And if there are strong winds higher up in the atmosphere, they can blow the top of a storm away from its base. This is known as wind shear. When there’s too much dry air or wind shear, storms do not usually intensify at all.
But Otis somehow managed to intensify rapidly despite there being some dry air and wind shear. “There’s something that we are missing,” says McNoldy.
It might be that some fringe events just cannot be predicted. “There is a limit of predictability in nature,” he says.
That would be a serious problem given that global warming is making it much more likely for tropical storms to intensify rapidly. This process usually happens over the open ocean, Heming says, but when it happens close to the coast as with Otis, the consequences can be dire.
“That sort of thing is a very real concern,” says McNoldy. “To be expecting a tropical storm when you maybe bring in some of your outdoor lawn furniture and wind chimes and things like that, and in the middle of the night a Category 5 hurricane is making landfall. That’s just unthinkable.”